The invention relates to systems for supplying fuel to internal combustion engines. It specifically relates to an electronic controller for controlling the air/fuel ratio of the mixture entering the cylinders of the engine and in particular such a controller which uses the output signal from an exhaust gas oxygen (EGO) sensor positioned on the exhaust path of the burned gases.
As a result of the ever increasing number of automobiles, there is a corresponding increase to the atmospheric pollution in cities and major highways, with the resulting risks for the population living in such a noxious environment.
FIG. 1 shows the relative proportions of the main constituents of burned gases on leaving an internal combustion engine as a function of the (A/F) air/fuel ratio of the carburetted or vaporized mixture which enters the engine cylinders. The constituents which are particularly pollutant are hydrocarbon, carbon monoxide and nitrogen oxides.
It is apparent from FIG. 1 that the pollution level of an engine is reduced when the latter is supplied with a lean mixture, i.e. whose air/fuel ratio exceeds 15:1 corresponding to the stoichiometric ratio indicated from the vertical broken line. It is known that carbon monoxide emission decreases when the air/fuel ratio increases as a result of the oxygen excess which ensures a more satisfactory combustion of the carburetted mixture. In practice, due to the imperfect air/fuel mixture and the short combustion period an air excess above the air/fuel stoichiometric ratio is found to be necessary. However, an air/fuel ratio above 18:1 constitutes an upper limit because, for various reasons, hydrocarbon emission increases again, ignition of the mixture becomes critical, the power supplied by the motor is reduced and the specific consumption increases, Thus, in order to comply with existing pollution standards and in particular those which are being prepared for future use it is necessary to take supplementary measures which generally consist of burning the residual noxious agents outside the engine in catalytic or noncatalytic reactors. It is apparent from what has been stated hereinbefore that to reduce the pollution level of motor vehicles to low values, the air/fuel ratio of the carburetted mixture which enters the engine cylinders must remain within narrow limits corresponding to a lean mixture and that to obtain pleasant and flexible driving conditions it is necessary to increase the richness of the carburetted mixture for certain engine running conditions.
In a conventional carburettion or vaporization system, it is not possible to control the air/fuel ratio of the carburetted mixture with the necessary precision, in view of manufacturing variations, the inevitable wear and variations to the operating conditions, particularly the engine temperature, ambient pressure, etc. Thus, electronic carburettion controllers have already been proposed which use the electrical output signal of an EGO sensor placed along the path of the engine exhaust gases to act on the fuel flow rate supplied to the carburettor. To obtain low carbon monoxide and hydrocarbon emissions, it is necessary to operate with air/fuel ratio values of approximately 16:1 to 18:1 which also correspond to a low specific consumption, but do not make it possible to obtain the maximum power from the engine and ensure adequate ignition of the mixture when the engine temperature is low.
A lean carburetted mixture is satisfactory when the engine temperature is then firmly established, and when the average speed of revolution and load of the engine are not too high. However, a rich mixture is necessary during the starting-up period of the engine, whilst operating at high load and when acceleration is needed. The known electronic carburettion controllers operate on a closed loop principle when the engine temperature is sufficiently high and with an average load. They change over to an open loop operating principle when acceleration is necessary or in the case of high loads, so as to operate like a conventional carburettion system. However, such a solution is far from being satisfactory because immediately the automatic correction provided by the closed loop controller is lost.
With a view to obviating this deficiency of carburettor controllers, it was proposed in French Patent Application 77.39842 to use a controller incorporating memory storage means making it possible to store the closed loop carburettor control value and then control the carburettor on the open loop principle on the basis of this stored value, increased by a quantity constituting a function of the engine temperature.